Most players have different concepts of what constitutes a "typical" battle. For some it may be 10 minifigs to a team, for others it might be a small army of figs, tanks, and attack helicopters. And I'm not even sure that everybody counts CP the same way. One player's 100cp vehicle can be another player's 30cp vehicle, depending on their house rules. I play with 300cp as a "casual" amount, because I like to use flyers, which are expensive.

typically the amount simply depends on how fast you can finish each turn and how much time you are willing to spend on a turn.
if turns take too long, you should have used a smaller army.
destroy/kill/remove minifigs from the game or stop using those superincredibly hard extra rules.
if they are too short for your liking, feel free to spawn extra stuff into your game.

Act first, then think, then try to find a way to cover up the horrible mess you made.
Si Hoc Legere Scis Nimium Eruditionis Habes

Rody wrote:if they are too short for your liking, feel free to spawn extra stuff into your game.

Yeah, sending in a random dropship full of soldiers when things get boring always works.

To find out what army size fits you best, try using the vehicle system from this game.
http://www.portablespacemuseum.com/brikwars/02/Instead of starting out with a set number of vehicles, the players can spawn a new vehicle each turn; it wouldn't be difficult to extend the same system to squads of infantry. When you feel you've reached an appropriate size/balance, just stop spawning units and play the rest of the game.